1. Field of the Invention
The present invention relates to a ball screw mechanism for displacing a nut member screwed onto a ball screw shaft in the axial direction driven by a rotary driving source.
2. Description of the Related Art
Conventionally, a screw section is engraved on an outer circumferential surface of a slide screw shaft which is rotatable under the rotary action of a rotary driving source such as a motor. A nut member of a metal material is screwed onto the screw section of the slide screw. A female screw section is formed in the nut member, for being screwed onto the screw section of the slide screw shaft. The displacement of the nut member in the rotational direction is restricted, and the rotational motion of the slide screw shaft is converted into the rectilinear motion of the nut member. Thus, the nut member can be displaced in the axial direction.
However, in the conventional slide screw as described above, when the nut member is displaced in the axial direction of the slide screw shaft, the displacement is effected while the screw section of the slide screw shaft is engaged with the female screw section of the nut member. Therefore, the frictional resistance appears between the screw section and the female screw section, so that transmission efficiency is low when the rotary motion of the slide screw shaft is converted into the rectilinear motion of the nut member.
Further, the slide screw shaft and the nut member are abraded due to the frictional resistance. Therefore, durability is lowered as a result of the use for many years.
On the other hand, in order to improve the durability and the transmission efficiency to transmit the driving force from a slide screw shaft to a nut member, a ball screw has been adopted, in which steel balls are interposed between a screw section and a female screw section, in place of the slide screw described above. In this case, it is necessary to grind the groove of the female screw section of the nut member on which the steel balls are arranged, in order to improve surface accuracy.
Further, in order to reduce the abrasion of the screw section of a ball screw shaft, it is necessary to heat the ball screw shaft for hardening the surface of the ball screw shaft. During the heat treatment, some strain may appear in relation to the shaft center of the ball screw shaft or the like due to the heat. Therefore, it is further necessary to correct the strain or the like of the ball screw shaft.
That is, when the ball screw is adopted, it is newly required, for example, to grind, heat, and correct the strain, as compared with the slide screw. Therefore, production steps are complicated, and the cost is raised accordingly.
Further, rolling resistance arises in the steel balls when the nut member is displaced in the axial direction with respect to the screw section of the ball screw shaft. For this reason, a lubricant is used for reducing the rolling resistance. However, it is necessary to periodically supply the lubricant, and the periodical maintenance operation is complicated.